NavigationController PushedView Controller dismisses entire NavigationController - swift - Programmatically

Question

I'm presenting a NavigationController modally like this:

let profileViewController = ProfileViewController(nibName: nil, bundle: nil)
        let navigationControllerProfile = UINavigationController(rootViewController: profileViewController)
        navigationControllerProfile.setNavigationBarHidden(true, animated: false)
        navigationControllerProfile.modalPresentationStyle = .fullScreen
        self.present(navigationControllerProfile, animated: true, completion: nil)

Whenever I push a Viewcontroller from this NavigationController:

let showCaseViewController = ShowcaseViewController()
        showCaseViewController.modalPresentationStyle = .overFullScreen
        self.navigationController?.pushViewController(showCaseViewController, animated: true)

The ViewController is pushed correctly but after 1 second the entire NavigationController is dismissed.

Answer 1

code to present view-controller with navigation

                let vc = storyboard?.instantiateViewController(withIdentifier: "firstvc") as! firstvc
                let navigation = UINavigationController(rootViewController: vc)
                vc.modalPresentationStyle = .fullScreen
                self.present(navigation, animated: true, completion: nil)

code for perform push operation inside presented view controller

            let vc = storyboard?.instantiateViewController(withIdentifier: "email") as! email
            self.navigationController?.pushViewController(vc!, animated: true)